The present invention relates to a switched power converter that comprises a piezoelectric transformer that works at constant frequency and regulates the power supplied at the output of the converter by means of disabling/enabling of the voltage applied to the input of the piezoelectric transformer.
One of the most significant characteristics of the piezoelectric devices is their noticeable frequency dependence; this fact is clearly illustrated by quartz oscillators, which are based on the piezoelectric effect in order to obtain determined frequencies, depending on various constructional parameters.
Within this context, transformers of the piezoelectric type do not constitute an exception and have a very marked impedance/frequency characteristic.
According to an equivalent circuit valid between the resonant and anti-resonant frequencies, the separation between the two frequencies is very reduced, of the order of a few tens of kilohertz.
This type of transformer finds employment in different DC/DC converter topologies, because the increase in power density and performance that are achieved are greater than those obtained with magnetic materials.
The power topologies based on incorporating transformers of the piezoelectric type try to include the parasitic elements in the actual topology, with the objective of minimising the number of components to be employed and that these elements distort the operation thereof to the least possible extent.
In this type of transformer there is a frequency that can be considered optimum from the point of view of the piezoelectric transformer gain, measured as the turns ratio thereof, efficiency, etc. If working at a constant frequency with this type of transformer, maximum advantage is taken of the excellent characteristics thereof.
Operating at constant frequency permits the optimisation of the piezoelectric transformer and of the remaining components of the power topology; on the other hand, it presents the serious drawback of how to regulate the power transferred to the secondary of the piezoelectric transformer.
It is known from the state of the art that the output voltage regulation is achieved by varying the frequency of the voltage applied at the input of the piezoelectric transformer. This type of regulation has some drawbacks, for example it is not possible to optimise the additional magnetic materials that have to be used, given that the working frequency is variable.
The short-circuit situation on the output of the piezoelectric transformer implies a hazardous situation for it, given that the frequency control will have a tendency to vary the frequency of the waveform applied at the input of the piezoelectric transformer, taking it to the maximum value possible; in this case, the input voltage to the piezoelectric transformer is boosted to really dangerous levels, and it is necessary to implement a sufficiently fast protection circuit to prevent the destruction of the piezoelectric transformer due to an overvoltage at the input.
It is necessary to implement protection against an open-circuit or no-load situation in the converter, given that in this situation, energy is being supplied to the piezoelectric transformer and none is being extracted from it; the result of this situation is an overvoltage on the input of the piezoelectric transformer and, just as in the previous case, it is necessary to implement another protection circuit to prevent the piezoelectric transformer under this undesired situation, with the consequent increase in component count.
The foregoing situations become still more complicated if it is taken into account that it is also necessary to detect when they vanish, or, in other words, when the main control has to retake command.
The regulation of a converter that incorporates a piezoelectric transformer is a problem, given that it is a non-linear device which greatly complicates the modelling thereof; the design of a regulation loop is therefore not a simple problem and shall be heavily dependent on the piezoelectric transformer and on the working conditions of the converter.
The narrow working margin of this type of device must again be stressed: the manufacturing spread can result in a converter working perfectly with a determined piezoelectric transformer whilst this same converter does not work correctly with another piezoelectric transformer from the same production batch (or the reverse situation), due to the spread in characteristics; in other words, the manufacturing spread in these converters should be practically nil.
All these inconveniences found in frequency control can be avoided if the control is carried out at a fixed frequency.
The switched converter of the invention includes, connected in cascade, a first rectifier module connected to a first capacitor that, in turn, is connected to an excitation module in order to produce a waveform that is applied across some input terminals of a piezoelectric transformer.
The piezoelectric transformer transforms the voltage waveform received into another voltage, which is rectified by a rectifier module and smoothed in a filter. The smoothed voltage corresponds to the output voltage of the switched converter.
A controller device governs the switching of the excitation module in order that the piezoelectric transformer works at constant frequency and regulates the power delivered to the output of the converter by means of disabling/enabling of the waveform applied across the input terminals of the piezoelectric transformer. When the piezoelectric transformer is not transferring energy from its input terminals to its output terminals, the output voltage of the converter is supplied by a second capacitor included in the output filter.
Thus, the input waveform to the piezoelectric transformer is modulated with a modulating waveform at low frequency; this modulating waveform is obtained by comparing the output voltage of the switched converter with a reference voltage to be reached.
The modulating waveform at low frequency is determined on the basis of the working conditions of the converter and of the output filter.